Chlorite

Chlorite, widespread group of layer silicate minerals occurring in both macroscopic and clay-grade sizes; they are hydrous aluminum silicates, usually of magnesium and iron. The name, from the Greek for “green,” refers to chlorite’s typical colour. Chlorites have a silicate layer structure similar to that in micas with compositions near (Mg,Fe,Al)3 (Si,Al)4O10(OH)2 and with brucitelike interlayers with compositions near (Mg,Fe,Al)3(OH)6. The overall composition is then (Mg,Fe,Al)6 (Si,Al)4O10(OH)8. The great many names found in older literature for chlorites with small variations of chemical composition are no longer used. The accepted names are: clinochlore (Mg-rich chlorite), chamosite (Fe-rich), nimite (Ni-rich), and pennantite (Mn-rich). Adjectival modifiers are used to indicate compositional variations. Cookeite (with lithium substituted for aluminum) is also a member of the chlorite group.

Chlorite from Calaveras county, California

B.M. Shaub

Chlorites characteristically occur as alteration products of other minerals. They are common rock-forming minerals in clastic sediments and in hydrothermally altered igneous rocks; chlorites are widespread and important constituents of such metamorphic rocks as greenschists or chlorite schists. For detailed structure and physical properties, seeclay mineral (table).

The greenschist facies was once considered the first major facies of metamorphism proper. The name comes from the abundance of the green mineral chlorite in such rocks. Because chlorite and muscovite are ubiquitous and because both exhibit a platy crystal habit, these rocks normally show a highly developed foliation and often exhibit strong metamorphic differentiation. They have been described...

The structure of the chlorite minerals consists of alternate micalike layers and brucitelike hydroxide sheets about 14 Å thick. Structural formulas of most trioctahedral chlorites may be expressed by four end-member compositions: